The present invention generally relates to video signal magnetic recording and reproducing systems, and more particularly to a video signal magnetic recording and reproducing system in which a recording video signal is recorded onto a magnetic tape after being compressed with respect to the time base, and a signal reproduced from the magnetic tape is expanded with respect to the time base to obtain a reproduced video signal.
Conventionally, so-called two-head system magnetic recording and reproducing apparatuses have been in wide practical use. In such a system, the video signal is alternately recorded onto and reproduced from tracks oblique with respect to the longitudinal direction of the magnetic tape, by two rotary heads. On the other hand, with the recent popularization of portable magnetic recording and reproducing apparatus, there is a demand for down-sizing the apparatus. Principally, if the relative linear speed between the magnetic head and the magnetic tape is constant, the diameter of a guide drum can be made smaller in a so-called one-head system using only one head. Hence, in order to satisfy the above demand for down-sizing the apparatus, it is preferable to use the one-head system.
As a type of this one-head system, there was a conventional system in which the magnetic tape is completely wound around the guide drum once in a spiral manner to form the shape of an .alpha., and the video signal is recorded onto and reproduced from the magnetic tape by the single rotary head. However, in an apparatus in which the tape is automatically drawn out from a tape cassette accommodating the tape, to automatically load the tape onto a predetermined tape path along the guide drum, it is difficult to wind the tape around the guide drum in the shape of .alpha.. Accordingly, the above conventional one-head system cannot be applied to the automatic tape loading type recording and reproducing apparatus.
When applying the one-head system to the automatic tape loading type recording and reproducing apparatus, the tape is guided by a pair of guide poles in the shape of an .OMEGA., as will be described later in conjunction with the drawings. Here, the pair of guide poles move while drawing out the tape from the tape cassette, and it becomes possible to automatically load the tape onto a predetermined tape path. However, when employing such a construction, a gap is inevitably formed between the pair of adjacent guide poles where the tape is not wound around the guide drum, in a state in which the loading of the tape is completed. Recording and reproduction cannot be performed with respect to the tape, during an interval in which the rotary head moves past the above gap. Thus, a period of lack of signal or signal lack is introduced in the reproduced signal during this interval.
As a counter measure against the above signal dropout, there is a method in which the period during which the signal is not recorded and reproduced is positioned in an interval corresponding to a vertical blanking period so that the signal lack is put at an inconspicuous position in the reproduced picture, and the lacking vertical synchronizing signal is compensated for upon reproduction. However, even by this method, lack of the video information is introduced at an upper or lower part of the reproduced picture. In addition, there is a disadvantage in that it was difficult to maintain synchronism upon compensation of the vertical synchronizing signal. In another attempt to minimize the above signal lack interval, there is a method in which the pair of guide poles are moved to positions exceedingly close to each other in the state where the loading of the tape is completed. However, even according to this method, there is a limit in closing the gap between the pair of guide poles due to limitations in the mechanical precision and the like, and the period in which the recording and reproduction cannot be performed is inevitably introduced. Furthermore, in this case, a large load is introduced with respect to the tape travel, because the angular range with which the tape is wound around the guide drum is large. Thus, there was a disadvantage in that an excessive force is applied on the tape.